Unsaturated monomeric formulations for the fabrication of polymeric waveguides

ABSTRACT

Monomeric formulations appropriate for creating self-propagating polymer optical waveguides, and methods for their fabrication, are disclosed. Multiple polymer waveguides can be fabricated simultaneously into a three-dimensional micro-truss structure, while avoiding significant polymerization outside the confines of the illuminated region. The formulations described to accomplish this controlled polymerization include species containing one or more unsaturated carbon-carbon bonds capable of being free-radical polymerized in the presence of photoinitiator and either a radical inhibitor species or a solvent, or both. The radical inhibitor and/or solvent are included to minimize heat buildup and thermal decomposition of initiator. This invention enhances the versatility of the chemistry by significantly increasing the number of chemical building blocks available for micro-truss fabrication.

PRIORITY DATA

This patent application is a divisional application of U.S. patentapplication Ser. No. 13/624,932 filed Sep. 23, 2012, now allowed, whichis hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention generally relates to the formation of polymericwaveguides and to monomeric formulations appropriate for suchwaveguides.

BACKGROUND OF THE INVENTION

An ordered three-dimensional (3D) microstructure is an ordered 3Dstructure at the micrometer or nanometer scale. 3D microstructures canbe manufactured from polymer materials such as polymer cellularmaterials. Currently, polymer cellular materials that are mass producedare created through various foaming processes, which all yield random(not ordered) 3D microstructures. 3D microstructures may also be knownas “micro-trusses.”

A stereolithography technique provides a method to build a 3Dmicrostructure in a layer-by-layer process. This process usuallyinvolves a platform that is lowered into a photo-monomer bath indiscrete steps. At each layer, a laser is used to scan over the area ofthe photo-monomer that is to be cured (i.e., polymerized) for thatparticular layer. Once the layer is cured, the platform is lowered, andthe process is repeated until the complete 3D structure is created.Modifications to the stereolithography technique have been developed toimprove the resolution by using laser optics and special resinformulations. Modifications have also been developed to decrease thefabrication time of the 3D structure by using a dynamic patterngenerator to cure an entire layer at once.

3D ordered polymer cellular structures have also been created usingoptical interference pattern techniques, also called holographiclithography; however, structures made using these techniques have anordered structure at the nanometer scale and the structures are limitedto the possible interference patterns.

A polymer optical waveguide can be formed in certain photopolymers thatundergo a refractive index change during the polymerization process.When a monomer that is photo-sensitive is exposed to light (e.g., UVlight) under the right conditions, the initial area of polymerization,such as a small circular area, will “trap” the light and guide it to thetip of the polymerized region due to the index of refraction change,further advancing that polymerized region. This process will continue,leading to the formation of a waveguide structure with approximately thesame cross-sectional dimensions along its entire length. Priortechniques to create polymer optical waveguides have only allowed one ora few waveguides to be formed.

The phenomenon of photopolymerizable resins and their use to createself-propagating polymer waveguides with a collimated beam of light isknown and multiple examples are provided, for example, in Kewitch andYariv, “Nonlinear optical properties for projection photolithography,”Appl Phys Lett. 68(4) 22 (1996); Kagami et al., “Light induced threedimensional optical waveguide,” Appl Phys Lett. 79(8) 1079 (2001); Shojiand Kawata, “Optically induced growth of fiber patterns in aphotopolymerizable resin,” Appl Phys Lett. 75(5) 737 (1999); andYamashita et al., “Fabrication of self-written waveguide inphotosensitive polyimide resin by controlling photochemical reaction ofphotosensitizer,” Appl Phys Lett. 85(18) 3962 (2004), which are eachincorporated by reference herein.

Previous versions of self-propagating polymer optical waveguide systemsdisclosed monomer formulations based on thiol-ene polymerization, as setforth in U.S. Pat. No. 8,017,193 issued Sep. 13, 2011 to Zhou andJacobsen at HRL Laboratories, LLC in Malibu, Calif., United States. U.S.Pat. No. 8,017,193 is hereby incorporated by reference herein in itsentirety for all purposes. This patent describes formation of apolymeric micro-truss structure using monomer formulations appropriatefor a thiol-ene system. This system produces high molecular weight by analternating reaction between a thyil radical reacting with a terminalunsaturated group followed by the reaction of a hydrogen radical withthe carbon-centered radical to regenerate a thyil radical and begin theprocess again.

While capable of producing a variety of polymeric structures, theavailable range of monomers along with structural variations andchemical functionality available to such a polymerization system islimited. In particular, the system disclosed in U.S. Pat. No. 8,017,193is relatively insensitive towards oxygen while growing the network,reducing the need for resin purification and processing. Also, a lowamount of heat is evolved upon growth of the network, preventing thermaldecomposition of photoinitiator and runaway polymerization outside theregions directly illuminated with UV light. However, the need for lowheat release restricts production of micro-truss systems to be limitedlargely to commercially available species containing multifunctionalthiol and unsaturated moieties.

See Hoyle and Bowman, “Thiolz-ene click chemistry,” Angew Chem Int Ed.49 1540 (2010) for an overview of the advantages, limitations, andapplications of thiol-ene chemistry; and Cramer et al., “Thiol-enepolymerization mechanism and rate limiting step changes for variousvinyl functional group chemistries,” Macromolecules 36 7964-7969 (2003)for a discussion of thiol-ene kinetics.

What are especially needed are improved monomer formulations and methodscapable of producing microstructures. These methods ideally would enablemicro-truss fabrication with formulations containing unsaturated speciesas the exclusive reactive group (i.e., without thiol), without runawaypolymerizations or inhibition of growing networks from oxygen. It isdesired to broaden the range of available monomer system available tothe micro-truss. The availability and range of chemical functionality ofpure unsaturated polymerization systems is much greater, thuspotentially broadening the synthetic choices available to themicro-truss developer.

Such advances could greatly improve the functionality and mechanicalproperties available to micro-truss structures. In addition, the cost ofmonomer resins can be decreased due to incorporation of inexpensivemonomers used in commodity plastics (e.g., methyl methacrylate) producedindustrially in high volume.

SUMMARY OF THE INVENTION

The present invention addresses the aforementioned needs in the art, aswill now be summarized and then further described in detail below.

In some variations, the invention provides a formulation for fabricatinga plurality of self-propagating polymer optical waveguides, saidformulation comprising:

(a) at least one starting molecule containing one or more unsaturatedcarbon-carbon bonds;

(b) a photoinitiator for initiating a free-radical polymerization ofsaid at least one starting molecule, wherein said photoinitiator ispresent in a photoinitiator concentration from about 0.01 wt % to about10 wt %; and

(c) a free-radical inhibitor for controlling said free-radicalpolymerization, wherein said free-radical inhibitor is present in aninhibitor concentration of from about 0.005 wt % to about 5 wt %,

wherein said formulation is essentially free of thiol groups; and

wherein said photoinitiator concentration and said inhibitorconcentration are selected to control formulation heat release,associated with polymerization, outside said polymer optical waveguides.

The starting molecule may be selected from the group consisting ofethylene, substituted olefins, halogenated olefins, 1,3-dienes, styrene,α-methyl styrene, vinyl esters, acrylates, methacrylates,acrylonitriles, acrylamides, N-vinyl carbazole, N-vinyl pyrolidone, andcombinations thereof in monomer or oligomer form. In some embodiments,the starting molecule is an acrylate selected from monoacrylates,diacrylates, triacrylates, tetraacrylates, pentaacrylates, orcombinations thereof. In some embodiments, the starting molecule is amethacrylate selected from monomethacrylates, dimethacrylates,trimethacrylates, tetramethacrylates, pentamethacrylates, orcombinations thereof.

The photoinitiator may be selected from the group consisting of2-hydroxy-2-methylpropiophenone, camphorquinone, benzophenone, benzoylperoxide, 2,2-dimethoxy-2-phenylacetophenone, azobisisobutyronitrile,and combinations thereof. In some embodiments, the photoinitiator iscapable of generating free radicals under exposure to light with awavelength selected from about 200 nm to about 500 nm, such as fromabout 365 nm to about 405 nm. The photoinitiator concentration may beselected from about 0.05 wt % to about 2 wt %, for example.

The free-radical inhibitor may be selected from the group consisting ofhydroquinone, methylhydroquinone, ethylhydroquinone,methoxyhydroquinone, ethoxyhydroquinone, monomethylether hydroquinone,propylhydroquinone, propoxyhydroquinone, tert-butylhydroquinone,n-butylhydroquinone, and combinations thereof. In some embodiments, theinhibitor concentration is selected from about 0.01 wt % to about 1 wt%.

In some embodiments, the formulation further comprises a solvent (suchas an aqueous solvent) or inert diluent. For example, a solvent or inertdiluent may be selected from the group consisting of cyclohexane,toluene, 1,4-dioxane, xylene, anisole, DMF, DMSO, water, ethanol,methanol, acetone, acetonitrile, chloroform, bulk monomer, derivatizedmonomer, and combinations thereof. In other embodiments, the formulationis free of solvent, or substantially free of solvent.

Other variations of the invention provide a formulation for fabricatinga plurality of self-propagating polymer optical waveguides, saidformulation comprising:

(a) at least one starting molecule containing one or more unsaturatedcarbon-carbon bonds;

(b) a photoinitiator for initiating a free-radical polymerization ofsaid at least one starting molecule, wherein said photoinitiator ispresent in a photoinitiator concentration from about 0.01 wt % to about10 wt %; and

(c) a non-aqueous solvent for controlling said free-radicalpolymerization,

wherein said formulation is essentially free of thiol groups; and

wherein said photoinitiator concentration and said solvent are selectedto control formulation heat release, associated with polymerization,outside said polymer optical waveguides.

In some embodiments of these other formulation variations, the startingmolecule may be selected from the group consisting of ethylene,substituted olefins, halogenated olefins, 1,3-dienes, styrene, α-methylstyrene, vinyl esters, acrylates, methacrylates, acrylonitriles,acrylamides, N-vinyl carbazole, N-vinyl pyrolidone, and combinationsthereof in monomer or oligomer form.

In some embodiments of these other formulation variations, thephotoinitiator may be selected from the group consisting of2-hydroxy-2-methylpropiophenone, camphorquinone, benzophenone, benzoylperoxide, 2,2-dimethoxy-2-phenylacetophenone, azobisisobutyronitrile,and combinations thereof.

In some embodiments of these other formulation variations, the solventmay be selected from the group consisting of cyclohexane, toluene,1,4-dioxane, xylene, anisole, DMF, DMSO, ethanol, methanol, acetone,acetonitrile, chloroform, and combinations thereof.

In some embodiments of these other formulation variations, afree-radical inhibitor is also present in an inhibitor concentration offrom about 0.005 wt % to about 5 wt %.

Variations of the invention also provide a viscoelastic micro-trusscomprising a polymerized form of a formulation that contains:

(a) at least one starting molecule containing one or more unsaturatedcarbon-carbon bonds;

(b) a photoinitiator for initiating a free-radical polymerization ofsaid at least one starting molecule, wherein said photoinitiator ispresent in a photoinitiator concentration from about 0.01 wt % to about10 wt %; and

(c) a free-radical inhibitor for controlling said free-radicalpolymerization, wherein said free-radical inhibitor is present in aninhibitor concentration of from about 0.005 wt % to about 5 wt %,

wherein said formulation is essentially free of thiol groups; and

wherein said photoinitiator concentration and said inhibitorconcentration are selected to control formulation heat release,associated with polymerization, outside said polymer optical waveguides.

Variations of the invention also provide a viscoelastic micro-trusscomprising a polymerized form of a formulation that contains:

(a) at least one starting molecule containing one or more unsaturatedcarbon-carbon bonds;

(b) a photoinitiator for initiating a free-radical polymerization ofsaid at least one starting molecule, wherein said photoinitiator ispresent in a photoinitiator concentration from about 0.01 wt % to about10 wt %; and

(c) a non-aqueous solvent for controlling said free-radicalpolymerization,

wherein said formulation is essentially free of thiol groups; and

wherein said photoinitiator concentration and said solvent are selectedto control formulation heat release, associated with polymerization,outside said polymer optical waveguides.

Still other variations enable and provide a method of fabricating apolymeric micro-truss, said method comprising:

(a) selecting at least one starting molecule containing one or moreunsaturated carbon-carbon bonds and not containing essentially any thiolgroups;

(b) selecting a photoinitiator for initiating a free-radicalpolymerization of said at least one starting molecule, wherein saidphotoinitiator is present in a photoinitiator concentration from about0.01 wt % to about 10 wt %;

(c) selecting a free-radical inhibitor for controlling said free-radicalpolymerization, wherein said free-radical inhibitor is present in aninhibitor concentration of from about 0.005 wt % to about 5 wt %;

(d) combining said starting molecule, said photoinitiator, and saidfree-radical inhibitor to generate a monomeric formulation;

(e) transferring said monomeric formulation into a mold;

(f) placing a mask over said monomeric formulation;

(g) exposing selected regions of said monomeric formulation, throughsaid mask, to beams of light, to induce free-radical polymerization;

(h) optionally removing unreacted monomeric formulation; and

(i) recovering a polymeric micro-truss comprising a polymerized form ofsaid monomeric formulation.

Yet other variations enable and provide a method of fabricating apolymeric micro-truss, said method comprising:

(a) selecting at least one starting molecule containing one or moreunsaturated carbon-carbon bonds and not containing essentially any thiolgroups;

(b) selecting a photoinitiator for initiating a free-radicalpolymerization of said at least one starting molecule, wherein saidphotoinitiator is present in a photoinitiator concentration from about0.01 wt % to about 10 wt %;

(c) selecting a non-aqueous solvent for controlling said free-radicalpolymerization;

(d) combining said starting molecule, said photoinitiator, and saidsolvent to generate a monomeric formulation;

(e) transferring said monomeric formulation into a mold;

(f) placing a mask over said monomeric formulation;

(g) exposing selected regions of said monomeric formulation, throughsaid mask, to beams of light, to induce free-radical polymerization;

(h) optionally removing unreacted monomeric formulation; and

(i) recovering a polymeric micro-truss comprising a polymerized form ofsaid monomeric formulation.

In some method embodiments, the photoinitiator concentration and saidinhibitor concentration are selected to control heat release associatedwith polymerization.

In some method embodiments, the method further comprises purging saidmonomeric formulation with an inert gas to exclude at least a portion ofdissolved oxygen present, if any, prior to step (g).

In step (g) of some method embodiments, said selected regions exposed tosaid beams of light are collectively from about 1% to about 50% of thetotal volume of said monomeric formulation, such as from about 5% toabout 20% of the total volume of said monomeric formulation.

In some method embodiments, the beams of light possess one or morewavelengths selected from about 200 nm to about 500 nm, such as fromabout 365 nm to about 405 nm. During step (g), said monomericformulation may be exposed to light with a power density from about 5mW/cm² to about 15 mW/cm², for example, and for a time period from about10 seconds to about 15 minutes, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a photographic image of the micro-truss of Example 1.

FIG. 1B is a photographic image of the micro-truss of Example 1.

FIG. 2 is a stress-strain curve for the micro-truss of Example 1.

FIG. 3 is a plot of % strain, modulus, and damping ability of themicro-truss as a function of temperature, in Example 1.

FIG. 4A is a photographic image of the micro-truss of Example 2.

FIG. 4B is a photographic image of the micro-truss of Example 2.

FIG. 5A is a photographic image of the micro-truss of Example 3.

FIG. 5B is a photographic image of the micro-truss of Example 3.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The compositions, apparatus, systems, and methods of the presentinvention will be described in detail by reference to variousnon-limiting embodiments.

This description will enable one skilled in the art to make and use theinvention, and it describes several embodiments, adaptations,variations, alternatives, and uses of the invention. These and otherembodiments, features, and advantages of the present invention willbecome more apparent to those skilled in the art when taken withreference to the following detailed description of the invention inconjunction with the accompanying drawings.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contextclearly indicates otherwise. Unless defined otherwise, all technical andscientific terms used herein have the same meaning as is commonlyunderstood by one of ordinary skill in the art to which this inventionbelongs.

Unless otherwise indicated, all numbers expressing conditions,concentrations, dimensions, and so forth used in the specification andclaims are to be understood as being modified in all instances by theterm “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the following specification andattached claims are approximations that may vary depending at least upona specific analytical technique.

The term “comprising,” which is synonymous with “including,”“containing,” or “characterized by” is inclusive or open-ended and doesnot exclude additional, unrecited elements or method steps. “Comprising”is a term of art used in claim language which means that the named claimelements are essential, but other claim elements may be added and stillform a construct within the scope of the claim.

As used herein, the phase “consisting of” excludes any element, step, oringredient not specified in the claim. When the phrase “consists of” (orvariations thereof) appears in a clause of the body of a claim, ratherthan immediately following the preamble, it limits only the element setforth in that clause; other elements are not excluded from the claim asa whole. As used herein, the phase “consisting essentially of” limitsthe scope of a claim to the specified elements or method steps, plusthose that do not materially affect the basis and novelcharacteristic(s) of the claimed subject matter.

With respect to the terms “comprising,” “consisting of,” and “consistingessentially of,” where one of these three terms is used herein, thepresently disclosed and claimed subject matter may include the use ofeither of the other two terms. Thus in some embodiments not otherwiseexplicitly recited, any instance of “comprising” may be replaced by“consisting of” or, alternatively, by “consisting essentially of.”

Free-radical polymerization of unsaturated species is a well-known fieldof polymer chemistry. Free-radical polymerization is typicallyaccomplished through generation of a radical species thermally,photochemically, or by ionizing radiation. Radicals (also referred to asfree radicals) are atoms, molecules, or ions with unpaired electrons. Anunsaturated compound is any organic compound that contains a derivativeof ethylene (H₂C═CH₂) with one or more hydrogen atoms replaced with someother group(s). Reaction of the radical species with an unsaturatedgroup will produce a larger molecule containing a carbon radical thatis, in turn, capable of reacting with another unsaturated monomer.Repetition of this reaction produces a growing linear chain, providedthere is adequate stabilization for the growing radical species.

Photopolymers undergo a refractive index change during thepolymerization process that can lead to the formation of polymer opticalwaveguides. If a monomer that is photo-sensitive is exposed to light(e.g., UV light) under the right conditions, the initial area ofpolymerization (e.g., a small circular area) will trap the light andguide it to the tip of the polymerized region, further advancing thatpolymerized region. This process will continue, leading to the formationof a waveguide structure with approximately the same cross-sectionaldimensions along its entire length. This phenomenon may be applied toform an interconnected pattern of self-propagating polymer waveguides.The polymeric waveguides may be configured into a micro-truss. Asintended herein, “micro-truss” is synonymous and interchangeable with“three-dimensional microstructure,” “3D microstructure,” and like terms.

Formation of self-propagating polymeric waveguides (see Background)relies on the ability to form high-molecular-weight networks in specificspatial regions of a resin mixture, while leaving remaining monomerlargely unreacted. Initiators that will dissociate under exposure to UVradiation provide a source of radical species to create polymericwaveguides. However, the rapid kinetics and exothermic nature offree-radical polymerizations often releases enough heat to thermallydecompose photolabile initiators. This process can lead to formation ofpolymer outside of the volume of resin directly illuminated with light.

In some variations, the present invention is premised on the recognitionof this problem and how it can be overcome to suppress runaway thermalpolymerization of a photopolymerizable resin. This is necessary in asystem such as a micro-truss, in contrast to a single waveguide. Inparticular, in a micro-truss, multiple self-propagating waveguidescomprising a large volume fraction of the overall resin may be formedsimultaneously and may generate heat sufficient to thermally decomposeinitiating species.

The origin of this problem is evident upon inspection of the kineticrate equation that governs free-radical polymerizations. The rate ofpolymerization is directly related to the rate of heat generated in aresin undergoing polymerization, since each addition of monomer speciesto a growing chain has a negative enthalpy change associated with it. Infree radical polymerizations the rate equation is:

$R_{p} = {k_{p}\sqrt{\frac{{fk}_{d}}{k_{t}}}\sqrt{I_{0}}M_{0}{\mathbb{e}}^{{- k_{d}}{t/2}}}$where I₀ and M₀ correspond to the initial concentration of monomer andinitiator in the resin, respectively. The terms f and k_(d) areassociated with the kinetics of initiator decomposition andcharacteristic of the specific initiator species chosen. For a giveninitiator, the rate of polymerization is determined by the kinetics ofpropagation of new monomer species to growing chains and termination ofactive chain ends, since R_(p) varies as k_(p)/√{square root over(k_(t))}.

For polymerization of unsaturated species in a free-radical manner,R_(p) is not constant over the course of the reaction. The rate oftermination k_(d) will markedly increase as the polymerizationprogresses. This is due to the fact that as molecular weight of a resinbegins to rise, the viscosity of the medium increases. The rate ofpropagation, k_(p), which depends on the rate of diffusion oflow-molecular-weight monomer, is largely unaffected during this process.However, the diffusion of high-molecular-weight chains is significantlyreduced. This reduction in diffusion limits the ability of growingradical species to find and react with each other through eithercombination or disproportionation mechanisms. As a result the rate oftermination, k_(t), decreases and the overall rate of polymerization,R_(p) increases as does the heat evolved. This phenomenon, known asautoacceleration or the Trommsdorff effect, can happen quite rapidly inbulk polymerization of neat resins.

Previous technologies associated with the formation of polymericwaveguides have not realized the need to address this problem. In thecase of fabrication of single-strand polymer waveguides, rapid heatbuildup and the resulting unwanted decomposition of photoinitiator arenot concerns. They are not concerns due to the low volume fraction ofilluminated resin to non-illuminated resin, as well as the ability ofthe large volume of surrounding resin to absorb heat and minimizethermally initiated polymerization outside of the directly illuminatedlight path (see Kewitch and Yariv, 1996; Kagami et al., 2001; Shoji andKawata, 1999; and Yamashita et al., 2004, each cited above).

In addition, the reports of polymer resin systems based on thiol-enepolymerization systems (e.g., U.S. Pat. No. 8,017,193 discussed above)appropriate for formation of self-propagating polymer waveguides are notas susceptible to the rapid rise of heating within the polymer resinupon UV illumination. There is a fundamentally different polymerizationmechanism and kinetics in thiol-ene polymerization systems.

In thiol-ene polymerization systems, the chain growth is closely relatedto a step-growth or polycondensation mechanism. The rate ofpolymerization is also proportional to 1/√{square root over (k_(t))} asin the pure free-radical case, and termination is determined bybimolecular reaction of two radical species. The strong dependence ofk_(t) on time due to changes in resin viscosity and diffusion does not,however, exist in a thiol-ene system.

In a pure free-radical system, at a given time during polymerizationthere is a bimodal distribution of high-molecular-weight chains andunreacted monomers with high molecular weight species forming at nearzero monomer conversion. Due to this behavior these systems are subjectto excessive cyclization, multiple cross-linking, microgelation, low gelpoint conversion, diffusion-controlled reactions and in general theformation of inhomogeneous networks. In the thiol-ene system, on theother hand, the growth mechanism provides a more even increase inmolecular weight of pre-gel species, leading to a steady growth oflow-molecular-weight species, and ultimately higher conversions and morehomogeneous gelled continuous networks (see Okay and Bowman, “Kineticmodeling of thiol-ene reactions with both step and chain growthaspects,” Macromolecular Theory and Simulations 2005, 14, 267-277). Thisremoves dependence of k_(t) on viscosity and allows modeling ofthiol-ene systems with the following rate equation:

$R_{p} = {\sqrt{\frac{R_{i}}{2k_{t}}}\left\lbrack {\frac{1}{\left( {k_{p\; 1}\left\lbrack {C = C} \right\rbrack} \right)^{2}} + \frac{1}{\left( {k_{p\; 2}\lbrack{SH}\rbrack} \right)^{2}} + \frac{1}{\left( {k_{p\; 1}{{k_{p\; 2}\left\lbrack {C = C} \right\rbrack}\lbrack{SH}\rbrack}} \right)^{2}}} \right\rbrack}^{- \frac{1}{2}}$using the assumption of equivalent termination kinetic parameters for avariety of vinyl species in thiol-ene systems (Cramer, 2003). In thisequation, [C═C] is the molar concentration of carbon-carbon double bondsand [SH] is the molar concentration of thiol groups.

Without being bound by any theories, with k_(t) less sensitive toviscosity increase or time as the reaction conversion proceeds, R_(p) inthiol-ene systems will not accelerate as found in free-radical systemswith their associated rise in heat generation and thermal decompositionof photoinitiator. The heat rise and photoinitiator decomposition infree-radical systems can lead to undesired runaway polymerizationoutside of the illuminated micro-truss regions, unless the principles ofthe present invention are applied.

Some variations of the present invention provide a formulation forforming self-propagating polymer optical waveguides, the formulationcomprising at least one molecule containing one or more unsaturatedcarbon-carbon bonds; an effective amount of a photoinitiator forinitiating a free-radical polymerization; an effective amount of afree-radical inhibitor for controlling the free-radical polymerization;and optionally a solvent or unreactive diluent.

The formulation is essentially free of thiol groups, in someembodiments, which means that the thiol concentration in the formulationis either zero or is insignificant in terms of final polymer properties.It should be recognized that thiol impurities may be present or that athiol-based additive may be introduced for reasons other than to serveas a monomer for polymerization.

The molecule containing one or more unsaturated carbon-carbon bonds maybe a monomer or an oligomer, i.e. it may be partially polymerizedalready. The unsaturated carbon-carbon bonds may be double bonds, triplebonds, or a combination thereof. Any molecule containing one or moreunsaturated carbon-carbon bonds, which is susceptible to free-radicalinitiation and thus capable of being polymerized, may be utilized.

According to some embodiments, appropriate monomers for free-radicalreactions can be selected from (but not limited to) ethylene,substituted olefins, halogenated olefins, 1,3-dienes, styrene, α-methylstyrene, vinyl esters, acrylates, diacrylates, triacrylates,methacrylates, acrylonitriles, acrylamides, N-vinyl carbazole, N-vinylpyrolidone, or combinations thereof. Exemplary monomers include, forexample, tris(2-hydroxyethyl)isocyanurate triacrylate, dipentaerythritolpentaacrylate, polyethylene glycol diacrylate, trimethylolpropanetriacrylate, and combinations or derivatives thereof.

Substituted olefins may include single atoms in place of hydrogen, suchas fluorine or chlorine, or groups such as alkyl groups, esters, aminegroups, hydroxyl groups, or cyano groups. One or more double bonds ortriple bonds can be present in the unsaturated molecules. Also, they cancontain different combination of these different multiple bonds.Different structures exhibit different reaction rates. Since variouscombinations of double or triple bonds may be used in the polymerizationprocess, polymer systems with very different physical properties can becreated.

While monomer species with a single unsaturated group are typicallycapable of forming linear chains, the formation of a solidhigh-molecular-weight material may be rapidly accelerated through theincorporation of monomer species with multiple polymerizable unsaturatedgroups, such as diacrylates, tricrylates, or larger acrylates.

The photoinitiator for initiating a free-radical polymerization is anycompound which can generate free radicals under light by eitherintramolecular bond cleavage or intermolecular hydrogen abstraction. Oneor more different types of photoinitiators may be used in thepolymerization process and usually result in different reaction rates.Examples include, but are not limited to,2-hydroxy-2-methylpropiophenone, camphorquinone, benzophenone, benzoylperoxide, 2,2-dimethoxy-2-phenylacetophenone, azobisisobutyronitrile, orcombinations thereof.

Photoinitiators may be present up to about 10 wt % of the total weightof the polymer. Preferably, the concentration of photoinitators is lessthan about 5 wt % or less than about 2 wt %, such as from about 0.01 wt% to about 0.1 wt %. Exemplary photoinitator concentrations includeabout 0.01 wt %, 0.02 wt %, 0.05 wt %, 0.075 wt %, 0.1 wt %, 0.2 wt %,and 0.5 wt % of the total weight of the polymer.

In a photoinitiator, free radicals are generated by the photoinitiatorwhen exposed to an appropriate wavelength of light, such as a wavelengthselected from about 200 nm to about 500 nm. Specific wavelengths may beselected, for example, from about 365-366 nm, 385 nm, 395 nm, and404-405 nm. The source of light may vary, such as fluorescent lamps(e.g. mercury arc lamps) or semiconductor light sources (e.g.,light-emitting diodes).

Free-radical inhibitors are components that aid in the suppression ofunwanted polymerization outside of light-illuminated regions. Afree-radical inhibitor added to the monomeric formulation reducesunwanted polymerization of the regions outside the optical waveguide.Polymerization of the unexposed regions outside the waveguide may occurfrom residual heat generated from the polymerization reaction or fromlight that leaks out of the waveguide during light exposure.

Exemplary materials for the free-radical inhibitor may be selected fromhydroquinone, methylhydroquinone, ethylhydroquinone,methoxyhydroquinone, ethoxyhydroquinone, monomethylether hydroquinone,propylhydroquinone, propoxyhydroquinone, tert-butylhydroquinone,n-butylhydroquinone, or combinations thereof.

The free-radical inhibitor concentration may selected to be between 0 wt% and 5 wt % by weight of the polymer, such as from about 0.005 wt % toabout 1 wt %, or about 0.01 wt %, 0.05 wt %, 0.1 wt %, 0.2 wt %, 0.4 wt%, 0.5 wt %, 0.75 wt %, or 0.9 wt % of the total weight of the polymer.

Dilution of polymerizable resin may be accomplished with an appropriatesolvent or diluent. In some embodiments, a solvent for the selectedmonomer is utilized. In other embodiments, there is no intentionalsolvent added (“neat polymerization”). The choice of solvent may varywidely, such as (for example) cyclohexane, toluene, 1,4-dioxane, xylene,anisole, DMF, DMSO, water, ethanol, methanol, acetone, acetonitrile,chloroform, bulk monomer, derivatized monomer, or combinations thereof.

Free-radical polymerizations of unsaturated monomers can be susceptibleto O₂ because it acts to scavenge (deactivate) active radical chainends. As a result, it may be desirable to remove dissolved O₂ from themonomer resin. This may be accomplished, for example, by evacuation ofthe resin mixture or delivering a continuous stream of inert gas (e.g.N₂, CO₂, or Ar) through the resin mixture for a fixed time period beforeexposure to light.

Some embodiments of this invention provide a method for preparation ofthe monomeric formulation. This method may include thoroughly mixing allmonomeric species, purging the resin formulation with inert gas toexclude all or substantially dissolved O₂, transferring the resin into amold, placing a mask over the resin, exposing selected regions of theresin through the mask to beams of collimated light, carrying outfree-radical polymerization, removing unreacted resin, and recovering apolymerized lattice structure.

The formation of a polymer waveguide requires an index of refractionchange between the liquid monomer and the solid polymer. To enableself-propagation of the polymer waveguide, the polymer should betransparent to the wavelength(s) of the light that will be used togenerate free radicals and induce polymerization. In addition to theserequirements, the reaction should stop, or significantly slow down, whenthe light exposure is off to avoid over-curing of the monomer thatsurrounds the polymer waveguide.

In some variations of this invention, a plurality of self-propagatingpolymer waveguides are created simultaneously using multiple collimatedbeads of light with a significantly higher volume fraction of the resinunder direct illumination compared to a single-strand polymer waveguide.This generates significantly greater heat per total resin volume. Thelevel of heat generated from the polymerization in the illuminatedregion can lead to undesirable thermal runaway polymerization outside ofthe illuminated regions and poor fidelity of the final structure withthe intended dimensions. In order to prevent unwanted thermally basedpolymerization in areas not under illumination, thermal decomposition ofinitiators should be mitigated. This can be accomplished by addition ofinhibitor species into the resin and/or by introduction of a solventsuch as water capable of absorbing heat and reducing temperature rise.

Embodiments of the present invention provide a polymeric system and amethod of using various monomeric formulations that enable the creationof a polymer waveguide suitable for the construction of micro-trusses orother 3D open-cellular polymer structures.

Some embodiments can create a polymer cellular material with an ordered3D microstructure by creating a pattern of self-propagating opticalwaveguides in an appropriate photopolymer. Forming single polymerwaveguides as well as patterning these polymer waveguides to form anordered 3D microstructure containing a plurality of self-propagatingpolymer waveguides, in some embodiments, is described in U.S. Pat. No.7,382,959 issued Jun. 3, 2008 to Jacobsen, which is hereby incorporatedby reference herein in its entirety.

According to some embodiments, a fixed light input (e.g., collimated UVlight) is used to cure (polymerize) polymer optical waveguides, whichcan self-propagate in a 3D pattern. The propagated polymer opticalwaveguides form an ordered 3D microstructure or micro-truss that can bepolymerized without anything moving during the formation process, toenable large-scale, inexpensive production.

In some embodiments, 3D microstructures or other ordered polymermicrostructures (including two-dimensional structures) may be designedfor a given application. Design parameters may include: the angle andpattern of the polymer waveguides with respect to one another; thepacking, or relative density of the resulting cellular structure (theopen volume fraction); and the cross-sectional shape and dimensions ofthe polymer waveguides.

A 3D polymer microstructure may be formed in the area exposed to theincident collimated light beam. Since the incident light and the monomerremain fixed with respect to one another during the formation of apolymer waveguide, the exposure area of the collimated beam(s) may bescanned over a larger surface area of monomer, leading to the formationof large-area structures. Once the polymer cellular structure is formedin the volume of monomer, the remaining unpolymerized material (monomer)is removed, leaving an open cellular polymer material that is theordered 3D microstructure. A solvent that will dissolve the monomer, butdoes not dissolve the polymer, may be used to aid in the monomerremoval. In certain embodiments, monomer may continuously be fed under afixed incident light pattern, created from a mask and collimated light.

Generally, multiple polymer waveguides may be created from multipleexposure times using a single collimated beam and a mask with a singleaperture and moving the collimated beam with respect to the mask betweeneach exposure. Alternatively, or additionally, multiple polymerwaveguides may be created from a single exposure time using multiplecollimated beams at different incident angles and a mask with a singleaperture.

A three-dimensional ordered polymer microstructure may be fabricatedfrom multiple polymer waveguides that are created from a mask withmultiple apertures. The shape and dimensions of the polymer waveguidesare dependent on the shape and dimensions of the aperture(s) in themask. The relative angles of the waveguides in the 3D structure aredependent on the incident angles of the collimated beam. The lengths ofthe individual polymer waveguides are dependent on the photopolymer andthe light source. The area of exposure of the collimated beam(s) and themonomer may be moved with respect to each other to create a structurethat is larger than the available exposure area.

The formed polymer cellular materials (3D microstructures) may be useddirectly, or as templates to form other materials with ordered 3Dmicrostructures, such as metals or ceramics. Because of the simplicityin the processing, as well as the versatility in material options,embodiments of the present invention have a wide range of applications,such as (but not limited to) lightweight structural materials;energy-absorbing materials; heat-transfer applications; deployablestructures (such as space structures); conformable core structures;acoustic damping; hook and loop attachments; compliant structures;optics for sub-micron waveguide formation; single body casting/net shapemanufacturing; alternate shapes for waveguide members; functionallygraded structures; heat exchanger/insulator structures; 3D battery/fuelcell structures; thermal switch structures; catalyst support structures;filtration/separation structures; wicking materials/moisture controlstructures; directional optical coupler/flexible display structures;distributed lighting structures; electrical interconnects; sensorsupports with high surface areas; biological growth templates; flexiblebody/reactive armors; stealth coatings; high friction/high wearsurfaces; waveguides for other energy sources; and flame-retardantfoams.

Certain embodiments of the invention will now be further described withreference to the examples, which by no means should be construed tolimit the invention. Examples 1, 2, and 3 demonstrate systems formedfrom purely unsaturated monomer resins. It is shown that with properselection of thiol-free compositions, clean polymeric micro-trussstructures may be successfully fabricated.

In each example, stress-strain and dynamic-frequency measurements areperformed on a TA Instruments Q-800 (TA Instruments, Delaware, US).Collimated UV light is generated by a 2 kW Mercury Arc Lamp (Bachur &Associates, California, US). Further details of the exposure set-up aredescribed in U.S. Pat. No. 7,382,959, which has been incorporated byreference. All resins are obtained from Sartomer USA, LLC (Pennsylvania,US) and used without further purification unless otherwise specified.2-2-dimethoxy-2 phenylacetophenone is a product of Sigma Aldrich(Wisconsin, US) and used as received.

Example 1

In this Example 1, tris(2-hydroxyethyl)isocyanurate triacrylate (19.73g, Cat#SR368) and dipentaerythritol pentaacrylate (21.0 g, Moles,Cat#SR399) are weighed and thoroughly mixed in a container.2-2-Dimethoxy-2-phenylacetophenone (20 mg, 0.05 wt %) is weighed out anddissolved in a small volume of acetone before being dispersed into theresin with vigorous mixing. The container is shielded from stray lightwith foil and placed in a vacuum chamber for 30 min. The monomerformulation is then loaded into a mold and a quartz mask with 225-μmapertures spaced at 2.25 mm in a square array is placed above the resin.The mask and resin combination are exposed to UV light with about 9mW/cm² at the resin surface for 30 sec. Following exposure, theunreacted resin is drained and the micro-truss structure rinsed withtoluene. The micro-truss is then placed in an 85° C. oven overnight todry.

FIGS. 1A and 1B show photographic images of the micro-truss fabricatedin this Example 1. FIG. 2 is a stress-strain curve for the micro-trussof Example 1. At ambient temperature, the sample is compressed at 100μm/min starting with an initial thickness of 3.4 mm. The run isterminated upon catastrophic collapse of the structure at approximately22% strain. FIG. 3 displays mechanical properties of the micro-truss(Example 1) as a function of temperature.

The initial sample is 16 mm (length)×16 mm (width)×3.2 mm (height) andhas an initial force of 0.1 N applied for a static stress of 0.39 kPa.The modulus (FIG. 3) is highly temperature-dependent, dropping over anorder of magnitude between ambient temperature and 45° C. The structureis quite viscoelastic at lower temperatures with tan(δ) of about 0.6,and decreasing above about 50° C., according to the data in FIG. 3.Tan(δ) is a measure of the damping ability of the material. Considerableexpansion is observed beyond 40° C. with the compressive strain droppingfrom 15% to 4% between ambient temperature and 120° C.

Example 2

In this Example 2, tris(2-hydroxyethyl)isocyanurate triacrylate (17.0 g,Cat#SR368) and trimethylolpropane triacrylate (17.0 g, Moles, Cat#SR351)are weighed and thoroughly mixed in a container.2-2-Dimethoxy-2-phenylacetophenone (20 mg, 0.05 wt %) is weighed out anddissolved in a small volume of acetone before being dispersed into theresin with vigorous mixing. Tertbutylhydroquinone (1.7 mg, 0.005 wt %)and tetraethylthiurian disulfide (1.7 mg, 0.005 wt %) are also dissolvedin a small volume of acetone. The container is shielded from stray lightwith foil and placed in a vacuum chamber for 30 min. The monomerformulation is then loaded into a mold and a quartz mask with 225-μmapertures spaced at 2.25 mm in a square array is placed above the resin.The mask and resin combination are exposed to UV light (about 9 mW/cm²at the resin surface) for 25 sec. Following exposure, the unreactedresin is drained and the micro-truss structure rinsed with toluene. Themicro-truss is then placed in an 85° C. oven overnight to dry.

FIGS. 4A and 4B show photographic images of the micro-truss fabricatedin this Example 2.

Example 3

In this Example 3, tris(2-hydroxyethyl)isocyanurate triacrylate (15.8 g,Cat#SR368), polyethylene glycol diacrylate (16.0 g, Moles, Cat#SR259),and dipentaerythritol pentaacrylate (8.4 g, Moles, Cat#SR399) areweighed and thoroughly mixed in a container.2-2-Dimethoxy-2-phenylacetophenone (20 mg, 0.05 wt %) is weighed out anddissolved in a small volume of acetone before being dispersed into theresin with vigorous mixing. The container is shielded from stray lightwith foil and placed in a vacuum chamber for 30 min. The monomerformulation is then loaded into a mold and a quartz mask with 225-μmapertures spaced at 2.25 mm in a square array is placed above the resin.The mask and resin combination are exposed to UV light (about 9 mW/cm²at the resin surface) for 40 sec. Following exposure, the unreactedresin is drained and the micro-truss structure rinsed with toluene. Themicro-truss is then placed in an 85° C. oven overnight to dry.

FIGS. 5A and 5B show photographic images of the micro-truss fabricatedin this Example 3.

In this detailed description, reference has been made to multipleembodiments and to the accompanying drawings in which are shown by wayof illustration specific exemplary embodiments of the invention. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatmodifications to the various disclosed embodiments may be made by askilled artisan.

Where methods and steps described above indicate certain eventsoccurring in certain order, those of ordinary skill in the art willrecognize that the ordering of certain steps may be modified and thatsuch modifications are in accordance with the variations of theinvention. Additionally, certain steps may be performed concurrently ina parallel process when possible, as well as performed sequentially.

All publications, patents, and patent applications cited in thisspecification are herein incorporated by reference in their entirety asif each publication, patent, or patent application were specifically andindividually put forth herein.

The embodiments, variations, and figures described above should providean indication of the utility and versatility of the present invention.Other embodiments that do not provide all of the features and advantagesset forth herein may also be utilized, without departing from the spiritand scope of the present invention. Such modifications and variationsare considered to be within the scope of the invention defined by theclaims.

What is claimed is:
 1. A method of fabricating a polymeric micro-trussusing a monomeric formulation, said method comprising: (a) selecting atleast one starting molecule containing one or more unsaturatedcarbon-carbon bonds and not containing essentially any thiol groups; (b)selecting a photoinitiator for initiating a free-radical polymerizationof said at least one starting molecule, wherein said photoinitiator ispresent in a photoinitiator concentration from about 0.01 wt % to about10 wt % based on said monomeric formulation; (c) selecting afree-radical inhibitor for controlling said free-radical polymerization,wherein said free-radical inhibitor is present in an inhibitorconcentration of from about 0.005 wt % to about 5 wt % based on saidmonomeric formulation; (d) combining said starting molecule, saidphotoinitiator, and said free-radical inhibitor to generate saidmonomeric formulation; (e) transferring said monomeric formulation intoa mold; (f) placing a mask over said monomeric formulation; (g) exposingselected regions of said monomeric formulation, through said mask, tobeams of light, to induce free-radical polymerization; (h) optionallyremoving unreacted monomeric formulation; and (i) recovering a polymericmicro-truss comprising a polymerized form of said monomeric formulation.2. The method of claim 1, wherein said photoinitiator concentration andsaid inhibitor concentration are selected to spatially control heatrelease associated with polymerization.
 3. The method of claim 1, saidmethod further comprising purging said monomeric formulation with aninert gas to exclude at least a portion of dissolved oxygen present, ifany, prior to step (g).
 4. The method of claim 1, wherein in step (g),said selected regions exposed to said beams of light are collectivelyfrom about 1% to about 50% of the total volume of said monomericformulation.
 5. The method of claim 4, wherein in step (g), saidselected regions exposed to said beams of light are collectively fromabout 5% to about 20% of the total volume of said monomeric formulation.6. The method of claim 1, wherein said beams of light possess one ormore wavelengths selected from about 200 nm to about 500 nm.
 7. Themethod of claim 6, wherein said beams of light possess one or morewavelengths selected from about 365 nm to about 405 nm.
 8. The method ofclaim 1, wherein during step (g), said monomeric formulation is exposedto light with a power density from about 5 mW/cm² to about 15 mW/cm². 9.The method of claim 1, wherein during step (g), said monomericformulation is exposed to light for a time period from about 10 secondsto about 15 minutes.
 10. A method of fabricating a polymeric micro-trussusing a monomeric formulation, said method comprising: (a) selecting atleast one starting molecule containing one or more unsaturatedcarbon-carbon bonds and not containing essentially any thiol groups; (b)selecting a photoinitiator for initiating a free-radical polymerizationof said at least one starting molecule, wherein said photoinitiator ispresent in a photoinitiator concentration from about 0.01 wt % to about10 wt % based on said monomeric formulation; (c) selecting a non-aqueoussolvent for controlling said free-radical polymerization; (d) combiningsaid starting molecule, said photoinitiator, and said solvent togenerate said monomeric formulation; (e) transferring said monomericformulation into a mold; (f) placing a mask over said monomericformulation; (g) exposing selected regions of said monomericformulation, through said mask, to beams of light, to inducefree-radical polymerization; (h) optionally removing unreacted monomericformulation; and (i) recovering a polymeric micro-truss comprising apolymerized form of said monomeric formulation.
 11. The method of claim10, wherein said photoinitiator concentration and said solvent areselected to spatially control heat release associated withpolymerization.
 12. The method of claim 10, said method furthercomprising purging said monomeric formulation with an inert gas toexclude at least a portion of dissolved oxygen present, if any, prior tostep (g).
 13. The method of claim 10, wherein in step (g), said selectedregions exposed to said beams of light are collectively from about 1% toabout 50% of the total volume of said monomeric formulation.
 14. Themethod of claim 13, wherein in step (g), said selected regions exposedto said beams of light are collectively from about 5% to about 20% ofthe total volume of said monomeric formulation.
 15. The method of claim10, wherein said beams of light possess one or more wavelengths selectedfrom about 200 nm to about 500 nm.
 16. The method of claim 15, whereinsaid beams of light possess one or more wavelengths selected from about365 nm to about 405 nm.
 17. The method of claim 10, wherein during step(g), said monomeric formulation is exposed to light with a power densityfrom about 5 mW/cm² to about 15 mW/cm².
 18. The method of claim 10,wherein during step (g), said monomeric formulation is exposed to lightfor a time period from about 10 seconds to about 15 minutes.